Optical waveguides are widely used for the transmission of information in which light carries information by modulating its intensity of phase. Example optical waveguides include optical fibers and waveguides in polymer films. While fiber optics can provide optical signal transmission over significant distances, waveguides in polymer films are of special interest since these waveguides can consist of nonlinear optical (NLO) polymer materials. Waveguides consisting of NLO polymers shift the phase of the guided light in response to electrical fields applied to the polymer thin film containing the waveguides. This provides an important means for applying modulation to waveguided light and devices containing waveguides using NLO polymers form an important class of electro-optical modulators.
Electro-optic (EO) modulators containing NLO polymers possess important advantages compared to other types of modulators. Advantages include their low dielectric constants for use in high frequency devices. Since the velocity of optical waves in polymers is very near that of modulating radio frequency waves, traveling wave modulators with long interaction lengths for efficient modulation are practical. Further, the ability to spin coat polymer films for the fabrication of waveguides using standard photolithography techniques provides fabrication compatibility with the semiconductor industry for integration of polymer waveguide devices with silicon integrated circuits. Additional features of NLO polymers include the ability to design and control optical nonlinearity through chemical synthesis and the potential for low cost, quantity production of a NLO polymer.
A buried channel waveguide in an NLO polymer typically consists of upper and lower layers of a cladding material having a lower refractive index with a middle layer having a higher refractive index. For horizontal confinement of the waveguide to a channel in the middle layer, the channel must be surrounded by a lower index on each side. Materials which do not exhibit appreciable EO effects result in "passive" channel waveguides, while the NLO polymers provide channel waveguides having EO activity after the polymer materials have been specially processed in an electric field to "pole" the materials. The processing of channel waveguides of various dimensions is critically important to the development of polymeric EO modulators and integration of these modulators with fiber optic transmission lines. The theory for developing polymer waveguides is well understood and depends upon the precise fabrication and control of refractive index gradients within polymer materials. Channel waveguides in polymeric materials can be fabricated by a variety of methods including (1) reactive ion etching and electron cyclotron resonance, (2) photochemical processing, (3) spatially selective poling, and (4) etching techniques using lasers or electron beams.
Reactive ion etching is a process used for semiconductor devices in which a radio frequency discharge creates reactive oxygen atoms which react with a material to etch it away. This process does not change the refractive index of a material, but can be used to etch ridges or troughs in a material using photoresist patterning. Haas, et. al. (U.S. Pat. No. 5,143,577), Nurse and Arrington (U.S. Pat. No. 5,263,111), Cho, et. al. (U.S. Pat. No. 5,332,690), and Haemmerle, et. al. (U.S. Pat. No. 5,439,782) all present methods of polymer channel waveguide fabrication in which etched trenches are filled by the subsequent deposition of a material having a differing refractive index, the etched features thus defining channel waveguides. Since the channel waveguide boundaries have abrupt refractive index changes where differing materials meet, the resulting waveguide boundaries are "step-index" boundaries. Light scattering is a very sensitive function of local refractive index discontinuities, and for this reason the optical quality of etched channel waveguides is very sensitive to the smoothness of etched features. In particular, Haas, et. al. (U.S. Pat. No. 5,143,577) recognized the requirements for extreme smoothness of etched waveguide walls in order to minimize optical propagation losses resulting from scattering at waveguide boundaries. Many researchers have continued with efforts to further increase etching smoothness and further reduce optical losses in channel waveguides. Recent developments in etching by electron cyclotron resonance provide greater control of the etching process and have demonstrated smoother surfaces to further reduce optical waveguiding losses as illustrated in the review article by L. R. Dalton et. al. "Synthesis and Processing of Improved Organic Second-Order Nonlinear Optical Materials for Applications in Photonics," Chem. Mater., Vol. 7, pp. 1075-1076, 1995.
Photochemical processing directly changes the refractive index of the material by changing either the .pi.-electron density or nuclear density as the result of a photochemically-induced conformational change. A variety of photochemical processes are known and include trans-to-cis isomerization, ring-opening reactions, keto-enol tautomerism, and interconversion between twisted charge transfer states. Trans-to-cis photo-isomerization and photo-induced polymerization have both been widely used to define optical waveguides in polymers. In particular, the trans-to-cis photo-isomerization of the azobenzene-containing NLO polymers has been used to create large changes in refractive index. As an example, the publication by Chen, et. al. "New Polymers with Large and Stable Second-Order Nonlinear Optical Effects," Macromolecules, Vol. 24, pp. 5421-5428, 1991, shows that the trans-to-cis conformation change of the azobenzene-based disperse red 19 (DR19) -containing polymers results in a large refractive index drop of approximately 0.2. In a publication by Shi et. al. entitled "Large Photoinduced Birefringence in an Optically Nonlinear Polyester Polymer," Appl. Phys. Lett., Vol. 59, pp. 2935-2937, 1991, this photoprocessing was employed with lithographic masks and standard UV exposure systems for photolithography to directly photoprocess channel waveguides in DR19-based NLO polymer films. Using a light field mask, the protected areas under the mask retain their high index to define channel waveguides relative to the exposed areas of the film which have a much lower index. The large refractive index gradients that have been produced through this processing with DR19-containing polymers, for example, are much too large for bounding the large single mode waveguides required for interfacing polymer EO devices with single mode optical fiber. Refractive index gradients need to be on the order of 0.005-0.01 for single mode operation of such large waveguides (5-10 .mu.m mode sizes). While polymers can be synthesized to contain lower concentrations of the azobenzene dye DR19 which will provide smaller refractive index gradients following UV photoprocessing, the lowered DR19 concentration results in corresponding loss of EO response of the polymer since the DR19 provides this functionality to the NLO polymer.
Photoprocessing can be used to create polymer channel waveguides in ways other than directly changing the refractive index of a material. Ashley (U.S. Pat. No. 5,265,185) and Malone, et. al. (U.S. Pat. No. 5,402,511) use lithographic exposure of ultraviolet (UV)-sensitive polymers to effect the selective polymerization of materials such as UV-curing epoxies. The unexposed polymer can be removed following lithography of a channel waveguide pattern using a suitable solvent to leave a polymer waveguide or a trench that can be filled with a higher index polymer. This photoprocessing approach restricts the choices of polymers for waveguiding or cladding layers to UV-curing polymers.
Thackera, et. al. (U.S. Pat. No. 5,006,285) applies spatially selective poling to define channel waveguides in NLO polymer films. The molecular reorientations from poling result in an increase in the refractive index in the poling direction and a decrease in the index orthogonal to the poling direction. This poling induced birefringence causes waveguides defined by poling to only support optical modes with the same polarization as the poling field. While the same electrodes can be used for poling and electro-optic modulation, this is not generally useful because the fine electrode patterns required for defining channel waveguides are very different from traveling wave or other electrodes required for high speed electro-optic modulation. A further complication is that for maximum EO response a material must be strongly poled. This leaves no control over the magnitude of the index change. Further complications of this approach arise from the requirement for "contact" poling, i.e. poling the film through the direct contact of electrodes. The high electric fields required for poling at elevated temperatures cause a number of serious problems including electrical shorts and breakdowns due to microscopic imperfections and inhomogeneities in the films as well as the migration of ions and chemical decomposition of the electrodes. Another difficulty is the presence of significant fringing fields beyond the edges of the electrode features being poled. This broadens and extends the poling field in the polymer layers.
Similar to reactive ion etching, laser etching can rapidly remove polymer material using intense laser light in the deep ultraviolet. Chiang and Haas (U.S. Pat. No. 5,106,211) and Nutt (U.S. Pat. No. 5,322,986) apply laser etching (also termed laser ablation or laser photo ablation) to the fabrication of channel waveguides in polymers. Optical propagation losses resulting from roughness of the resulting waveguide walls are a serious concern with this technique, as with other etching approaches. Also, the local temperatures must be carefully controlled with this processing to avoid damaging the polymer or depoling the polymer in the laser etch process. This technique has been most suitable for the fabrication of large multimode (e.g., 10 .mu.m and larger) waveguide structures.
One of the most serious shortcomings of all of the polymer waveguide fabrication processes described above is the inability to readily fabricate low optical loss devices which accommodate multiple sizes of single mode waveguides in a single device. The patents and processes described above have two-dimensional patterning capability in the plane of the film, but very limited or no processing flexibility in the depth of the film. For example, the etching processes proceed as uniformly as possible over the substrate surface to the same depth with the patterning capability being two dimensional. Previous photoprocessing, for example, has used strongly absorbed light resulting in large refractive index gradients. These exposures have been applied uniformily over the substrates with the large index changes conducted deeply into and through the polymer films.
For an EO polymer waveguiding device to have low optical coupling losses to single mode optical fibers, a large mode size of 5-10 microns is required for typical infrared communications wavelengths of 1.3-1.5 .mu.m. A film thickness for the channel waveguide of 5-10 .mu.m can correspondingly be used to achieve large mode sizes, but this requires small refractive index gradients (.DELTA.n) at the channel waveguide boundaries to be single mode. As a consequence of the small .DELTA.n, the guided mode extends significantly into the cladding layers. Since metals exhibit significant absorption of visible and infrared light, this requires the cladding layers to also be quite thick in order to insulate the mode from the modulator's electrodes to minimize this source of optical loss. Many researchers have succeeder in fabricating such single mode channel waveguides. Excellent, low loss coupling to optical fiber has been demonstrated as well as low loss waveguides. However, such waveguides have large modulator electrode spacings as a result of the thick NLO polymer film and the thick cladding layers. This results in poor EO modulation and the requirement for large drive voltages (e.g., 10's of volts rather than a few volts). Increasing the length of the modulator reduces the drive voltages, but this is not a practical solution for very high frequency rf applications, maintenance of low optical loss, fabrication simplicity and integration with semiconductor electronics.
Optimizing waveguide dimensions for the modulator instead of the fiber interface, very small single mode waveguides in the range of 1.5-2.0 .mu.m can be fabricated using thin NLO polymer films. A much larger .DELTA.n is required for a waveguide boundary, but the single mode penetration of the cladding is also small. This permits small cladding thicknesses. Small electrode spacings and low drive voltages are achieved as a result of this approach, but now there is a large mode size mismatch if such a device is to be coupled to optical fiber. This results in very large optical insertion losses which make such a device impractical.
Low loss waveguide tapers can interconnect large mode sizes for coupling to optical fibers with small modes for efficient modulators. The need for such waveguide tapers has been increasingly appreciated. Malone, et. al. (U.S. Pat. No. 5,402,511) and Haemmerle, et. al. (U.S. Pat. No. 5,439,782) describe UV photo polymerization processes in which waveguide tapers are patterned. While Malone, et. al. (U.S. Pat. No. 5,402,511) recognizes the importance of three-dimensional patterning of a waveguide taper within the polymer film, their processing is inherently two-dimensional. Either a sloped substrate or a non-uniform polymer film deposition must be used to achieve variation in the waveguide thickness. The fabrication of slopes in the substrate introduces additional processing steps which may be incompatible with other substrate processing, can introduce microscopic surface roughness resulting in increased optical propagation losses, and can increase fabrication complexity and cost. Nonuniform polymer thin film depositions are difficult to control and standardize, and are generally avoided in the design of fabrication processes. Haemmerle, et. al. (U.S. Pat. No. 5,439,782) uses thermal processing of a two dimensional taper pattern in a meltable polymer. This processing redistributes the polymer to a nonuniform thickness that varies with width to result in a tapered thickness. This approach requires meltable polymers, but the majority of NLO polymer materials developed over the last 5 years which show the greatest promise for sufficiently large, thermally stable, optical nonlinearities are polymers having high densities of strongly nonlinear chromophores (the polymer component responsible for the optical nonlinearity) and the polymers are "lattice hardened" through crosslinking as reviewed by L. R. Dalton, et. al. in "Synthesis and Processing of Improved Organic Second-Order Nonlinear Optical Materials for Applications in Photonics," Chem. Mater., Vol. 7, pp. 1069-1074, 1995. As a result of this crosslinking, these polymers do not melt. Further, the increasing use of chromophores have large optical nonlinearities also results in non-meltable polymers because many of these chromophores undergo decomposition near their melting points.
The fabrication methods discussed above have been shown to address individual aspects of the requirements for small, very high frequency EO polymer devices having low optical insertion loss and low drive voltages, but it has been impractical to realize these simultaneously in a single device fabrication because of the large difference in mode sizes that are optimum for fiber coupling and for EO modulator design. This requires spatial variations in waveguide fabrication in the vertical dimension in addition to control in the plane of the films. A fabrication method which permits three-dimensional control of refractive index gradients within films is needed so that single mode channel waveguides of different sizes can be buried within NLO polymer films and connected by low loss, three dimensional waveguide tapers. This would allow large single mode waveguides to be fabricated at the device periphery for coupling to fiber optics. These waveguides would transition to very small single mode waveguides in a high efficiency EO modulator to result in an EO device having both low insertion loss and low drive voltages.